2.2.3.2 Agar
Synonyms Agar-agar; Gelose; Japan-agar; Chinese-isinglass; Bengal isinglass; Ceylon isinglass; Layor carang; Vegetable gelatin.
Biological Source Agar is the dried hydrophilic colloidal polysaccharide complex extracted from the agarocytes of algae belonging to the class Rhodophyceae. It is also obtained as the dried gelatinous substance from Gelidium amansii belonging to the family Gelidaceae and several other species of red algae, such as Gracilaria (family: Gracilariaceae) and Pterocladia (Gelidaceae). The predominant agar-producing genera are, namely; Gelidium; Gracilaria; Acanthopeltis; Ceramium and Pterocladia.
Geographical Source Agar is largely produced in Japan, Australia, India, New Zealand, and USA. It is also found in Korea, Spain, South Africa and in the Coastal regions of Bay of Bengal (India) together with Atlantic and Specific Coast of USA.
Preparation It is an usual practice in Japan where the red-algae is cultivated by placing poles or bamboos spread in the ocean which will serve as a support and shall augment the growth of algae on them. During the months of May and October the poles are removed and the algae are carefully stripped off from them. The fresh seaweed thus collected is washed thoroughly in water and subsequently extracted in digestors containing hot solution of dilute acid (1 portion of algae to 60 portions of diluted acid). The mucilagenous extract is filtered through linen while hot and collected in large wooden troughs to cool down to ambient temperature so as to form solid gel. The gel is mechanically cut into bars and passed through a wire netting to form strips. The moisture from the strips is removed by successive freezing and thawing* and finally sun dried and stored as thin agar strips.
Alternatively, the mass of gel if frozen and subsequently thawed and the dried agar is obtained by vaccum filtration. The crude agar is usually formed as flakes which can be powdered and stored accordingly.
* To bring down to room temperature from –20 to –30oC.
Description
Colour : Yellowish white or Yellowish grey
Odour : Odourless
Taste : Bland and mucilaginous
Shape : It is available in different shapes, such as: bands, strips, flakes, sheets and coarse powder
Size : Bands: width = 4cm; Length = 40 to 50 cms
Sheets: Width = 10-15cm; Length = 45 to 60 cms
Strips: Width = 4mm; Length = 12 to 15 cms
India produces about 250 MT of good quality agar using Galidiella accrosa as the raw material. It is insoluble in cold water in organic solvents. It readily dissolves in hot solutions and it forms a translucent solid mass which characteristic is very useful in microbiology for carrying out the Standard Plate Count.
Chemical Constituents Agar can be separated into two major fractions, namely: (a) Agarose-a neutral gelling fraction; and (b) Agaropectin—a sulphated non-gelling fraction. The former is solely responsible for the gel-strength of agar and consists of (+) –galactose and 3,6-anhydro-(–)-galactose moieties; whereas the latter is responsible for the viscosity of agar solutions and comprises of sulphonated polysaccharide wherein both uronic acid and galactose moieties are partially esterified with sulphuric acid. In short, it is believed to be a complex range of polysaccharide chains having alternating α–(1→3) and β–(1-4) linkages and varying total charge content.
Chemical Tests
1. It gives a pink colouration with Ruthenium Red solution.
2. A 1.5-2.0% (w/v) solution of agar when boiled and cooled produces a stiff-jelly.
3. Prepare a 0.5%(w/v) solution of agar and add to 5 ml of it 0.5 ml of HCl, boil gently for 30 minutes and divide into two equal portions:
(a) To one portion add BaCl2 solution and observe a slight whitish precipitate due to the formation of BaS04 (distinction from Tragacanth), and
(b) To the other portion add dilute KOH solution for neutralization, add 2 ml of Fehling’s
solution and heat on a water bath. The appearance of a brick red precipitate confirms the presence of galactose.
Substituents/Adulterants Gelatin and isinglass are usually used as substituents for agar.
Uses
1. It is used in making photographic emulsions.
2. It is also employed as a bulk laxative.
3. It is extensively used in preparing gels in cosmetics.
4. It is widely used as thickening agent in confectionaries and dairy products.
5. It is used in the production of ointments and medicinal encapsulations.
6. In microbiology, it is employed in the preparation of bacteriological culture media.
7. It is used for sizing silks and paper.
8. It finds its enormous usage in the dyeing and printing of fabrics and textiles.
9. It is also used as dental impression mould base.
10. It is employed as corrosion inhibitor.
